The frequency/wave vector dispersion relation ${\nu }_j\left(\mathbf{q}\right)$ for the normal modes of vibration of pure potassium iodide at 90°K has been measured by inelastic neutron scattering techniques. The experiments were performed with both a time-of-flight machine and a triple-axis crystal spectrometer. The results obtained from the latter spectrometer have been analyzed in terms of various interionic-force models. A satisfactory fit to the results is provided by an eleven-parameter dipole approximation model, which allows for the polarizability of both ions. Comparison of normal-mode frequencies calculated from this model with those observed in the time-of-flight experiments shows generally good agreement. The frequency distribution function for the normal modes, computed from the best-fit model, displays a well-defined energy gap, from 8.65 to 11.85 meV, separating the acoustic and optic modes. The moments of this function are in excellent agreement with values derived from heat-capacity data. By assuming an exponential form for the nearest-neighbor short-range force constants, the temperature variation of the thermal expansion, and the frequency shift and inverse lifetime of the transverse optic mode of very long wavelength, have been calculated and compared with the available experimental results. The occurrence of localized impurity modes in the energy gap in the frequency distribution for potassium iodide doped with potassium nitrite is briefly discussed.

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